根据我的理解,async和await所做的主要事情之一是使代码易于编写和阅读-但使用它们是否等于生成后台线程来执行长时间的逻辑?

我目前正在尝试最基本的例子。我内联添加了一些注释。你能给我解释一下吗?

// I don't understand why this method must be marked as `async`.
private async void button1_Click(object sender, EventArgs e)
{
    Task<int> access = DoSomethingAsync();
    // task independent stuff here

    // this line is reached after the 5 seconds sleep from 
    // DoSomethingAsync() method. Shouldn't it be reached immediately? 
    int a = 1; 

    // from my understanding the waiting should be done here.
    int x = await access; 
}

async Task<int> DoSomethingAsync()
{
    // is this executed on a background thread?
    System.Threading.Thread.Sleep(5000);
    return 1;
}

当前回答

异步&等待简单的解释

简单的类比

一个人可能会等早班火车。这就是他们所做的一切,因为这是他们目前正在执行的主要任务。(同步编程(你通常做的事情!))

另一些人可能在等早班火车的时候,抽着烟,喝着咖啡。(异步编程)

什么是异步编程?

异步编程是指程序员选择在与主线程执行分开的线程上运行一些代码,然后在执行完成时通知主线程。

async关键字实际上做什么?

在方法名前加上async关键字,例如

async void DoSomething(){ . . .

允许程序员在调用异步任务时使用await关键字。这就是它所做的。

为什么这很重要?

在许多软件系统中,主线程专门用于与用户界面相关的操作。如果我正在运行一个非常复杂的递归算法,需要5秒才能在我的计算机上完成,但我在主线程(UI线程)上运行这个,当用户试图单击我的应用程序上的任何东西时,它将出现冻结,因为我的主线程已经排队,目前正在处理太多的操作。因此,主线程无法处理从鼠标点击到从按钮点击运行的方法。

什么时候使用Async和Await?

在不涉及用户界面的情况下,最好使用异步关键字。

假设你正在编写一个程序,允许用户在手机上画草图,但是每隔5秒它就会在互联网上查看天气。

我们应该等待呼叫,轮询每5秒呼叫一次网络以获取天气,因为应用程序的用户需要保持与移动触摸屏的交互以绘制漂亮的图片。

如何使用Async和Await

从上面的例子,下面是一些如何编写它的伪代码:

    //ASYNCHRONOUS
    //this is called using the await keyword every 5 seconds from a polling timer or something.

    async Task CheckWeather()
    {
        var weather = await GetWeather();
        //do something with the weather now you have it
    }

    async Task<WeatherResult> GetWeather()
    {

        var weatherJson = await CallToNetworkAddressToGetWeather();
        return deserializeJson<weatherJson>(weatherJson);
    }

    //SYNCHRONOUS
    //This method is called whenever the screen is pressed
    void ScreenPressed()
    {
        DrawSketchOnScreen();
    }

附加说明-更新

我忘了在我最初的笔记中提到,在c#中,你只能等待被包裹在任务中的方法。例如,你可以等待这个方法:

// awaiting this will return a string.
// calling this without await (synchronously) will result in a Task<string> object.
async Task<string> FetchHelloWorld() {..

你不能等待不是任务的方法,像这样:

async string FetchHelloWorld() {..

请随意查看Task类的源代码。

其他回答

下面是通过打开对话框读取excel文件的代码,然后使用async和等待运行异步代码,从excel逐行读取并绑定到网格

namespace EmailBillingRates
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            lblProcessing.Text = "";
        }

        private async void btnReadExcel_Click(object sender, EventArgs e)
        {
            string filename = OpenFileDialog();

            Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(filename);
            Microsoft.Office.Interop.Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
            Microsoft.Office.Interop.Excel.Range xlRange = xlWorksheet.UsedRange;
            try
            {
                Task<int> longRunningTask = BindGrid(xlRange);
                int result = await longRunningTask;

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message.ToString());
            }
            finally
            {
                //cleanup  
               // GC.Collect();
                //GC.WaitForPendingFinalizers();

                //rule of thumb for releasing com objects:  
                //  never use two dots, all COM objects must be referenced and released individually  
                //  ex: [somthing].[something].[something] is bad  

                //release com objects to fully kill excel process from running in the background  
                Marshal.ReleaseComObject(xlRange);
                Marshal.ReleaseComObject(xlWorksheet);

                //close and release  
                xlWorkbook.Close();
                Marshal.ReleaseComObject(xlWorkbook);

                //quit and release  
                xlApp.Quit();
                Marshal.ReleaseComObject(xlApp);
            }

        }

        private void btnSendEmail_Click(object sender, EventArgs e)
        {

        }

        private string OpenFileDialog()
        {
            string filename = "";
            OpenFileDialog fdlg = new OpenFileDialog();
            fdlg.Title = "Excel File Dialog";
            fdlg.InitialDirectory = @"c:\";
            fdlg.Filter = "All files (*.*)|*.*|All files (*.*)|*.*";
            fdlg.FilterIndex = 2;
            fdlg.RestoreDirectory = true;
            if (fdlg.ShowDialog() == DialogResult.OK)
            {
                filename = fdlg.FileName;
            }
            return filename;
        }

        private async Task<int> BindGrid(Microsoft.Office.Interop.Excel.Range xlRange)
        {
            lblProcessing.Text = "Processing File.. Please wait";
            int rowCount = xlRange.Rows.Count;
            int colCount = xlRange.Columns.Count;

            // dt.Column = colCount;  
            dataGridView1.ColumnCount = colCount;
            dataGridView1.RowCount = rowCount;

            for (int i = 1; i <= rowCount; i++)
            {
                for (int j = 1; j <= colCount; j++)
                {
                    //write the value to the Grid  
                    if (xlRange.Cells[i, j] != null && xlRange.Cells[i, j].Value2 != null)
                    {
                         await Task.Delay(1);
                         dataGridView1.Rows[i - 1].Cells[j - 1].Value =  xlRange.Cells[i, j].Value2.ToString();
                    }

                }
            }
            lblProcessing.Text = "";
            return 0;
        }
    }

    internal class async
    {
    }
}

在一个简单的控制台程序中显示上述解释:

class Program
{
    static void Main(string[] args)
    {
        TestAsyncAwaitMethods();
        Console.WriteLine("Press any key to exit...");
        Console.ReadLine();
    }

    public async static void TestAsyncAwaitMethods()
    {
        await LongRunningMethod();
    }

    public static async Task<int> LongRunningMethod()
    {
        Console.WriteLine("Starting Long Running method...");
        await Task.Delay(5000);
        Console.WriteLine("End Long Running method...");
        return 1;
    }
}

输出为:

Starting Long Running method...
Press any key to exit...
End Long Running method...

因此,

Main通过TestAsyncAwaitMethods启动长时间运行的方法。这立即返回,而不停止当前线程,我们立即看到'按任何键退出'消息 在此期间,LongRunningMethod一直在后台运行。一旦它完成,来自Threadpool的另一个线程拾取该上下文并显示最终消息

因此,没有线程被阻塞。

这里有一个快速的控制台程序,让那些遵循。TaskToDo方法是你想让它异步的长期运行的方法。让它以异步方式运行是由TestAsync方法完成的。test loops方法只是运行TaskToDo任务,并异步运行它们。你可以在结果中看到这一点,因为它们在每次运行中完成的顺序不同——当它们完成时,它们会报告给控制台UI线程。简单,但我认为简单的例子比复杂的例子更好地揭示了模式的核心:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace TestingAsync
{
    class Program
    {
        static void Main(string[] args)
        {
            TestLoops();
            Console.Read();
        }

        private static async void TestLoops()
        {
            for (int i = 0; i < 100; i++)
            {
                await TestAsync(i);
            }
        }

        private static Task TestAsync(int i)
        {
            return Task.Run(() => TaskToDo(i));
        }

        private async static void TaskToDo(int i)
        {
            await Task.Delay(10);
            Console.WriteLine(i);
        }
    }
}

异步/等待

实际上,Async / Await是一对关键字,它们只是用于创建异步任务回调的语法糖。

举个例子:

public static void DoSomeWork()
{
    var task = Task.Run(() =>
    {
        // [RUNS ON WORKER THREAD]

        // IS NOT bubbling up due to the different threads
        throw new Exception();
        Thread.Sleep(2000);

        return "Hello";
    });

    // This is the callback
    task.ContinueWith((t) => {
        // -> Exception is swallowed silently
        Console.WriteLine("Completed");

        // [RUNS ON WORKER THREAD]
    });
}

上面的代码有几个缺点。错误不会传递,而且很难阅读。 但是Async和Await来帮助我们:

public async static void DoSomeWork()
{
    var result = await Task.Run(() =>
    {
        // [RUNS ON WORKER THREAD]

        // IS bubbling up
        throw new Exception();
        Thread.Sleep(2000);

        return "Hello";
    });

    // every thing below is a callback 
    // (including the calling methods)

    Console.WriteLine("Completed");
}

Await调用必须在Async方法中。这有一些优点:

返回Task的结果 自动创建回调 检查错误并让它们在callstack中冒泡(只适用于callstack中的无等待调用) 等待结果 释放主线程 在主线程上运行回调 使用线程池中的工作线程执行任务 使代码易于阅读 还有更多

注意:Async和Await用于异步调用时不做这些。为此必须使用任务库,如Task. run()。

下面是等待和无等待解决方案之间的比较

这是一个非异步解决方案:

public static long DoTask()
{
    stopWatch.Reset();
    stopWatch.Start();

    // [RUNS ON MAIN THREAD]
    var task = Task.Run(() => {
        Thread.Sleep(2000);
        // [RUNS ON WORKER THREAD]
    });
    // goes directly further
    // WITHOUT waiting until the task is finished

    // [RUNS ON MAIN THREAD]

    stopWatch.Stop();
    // 50 milliseconds
    return stopWatch.ElapsedMilliseconds;
}

这是async方法:

public async static Task<long> DoAwaitTask()
{
    stopWatch.Reset();
    stopWatch.Start();

    // [RUNS ON MAIN THREAD]

    await Task.Run(() => {
        Thread.Sleep(2000);
        // [RUNS ON WORKER THREAD]
    });
    // Waits until task is finished

    // [RUNS ON MAIN THREAD]

    stopWatch.Stop();
    // 2050 milliseconds
    return stopWatch.ElapsedMilliseconds;
}

实际上,你可以不使用await关键字而调用async方法,但这意味着这里的任何异常都会在释放模式下被吞噬:

public static Stopwatch stopWatch { get; } = new Stopwatch();

static void Main(string[] args)
{
    Console.WriteLine("DoAwaitTask: " + DoAwaitTask().Result + " ms");
    // 2050 (2000 more because of the await)
    Console.WriteLine("DoTask: " + DoTask() + " ms");
    // 50
    Console.ReadKey();
}

Async和Await并不用于并行计算。它们用于不阻塞主线程。当涉及asp.net或Windows应用程序时,由于网络调用阻塞主线程是一件糟糕的事情。如果你这样做,你的应用程序将得不到响应,甚至崩溃。

查看微软文档以获得更多的例子。

这里所有的答案都使用Task.Delay()或其他内置的异步函数。但下面是我的例子,没有使用这些async函数:

// Starts counting to a large number and then immediately displays message "I'm counting...". 
// Then it waits for task to finish and displays "finished, press any key".
static void asyncTest ()
{
    Console.WriteLine("Started asyncTest()");
    Task<long> task = asyncTest_count();
    Console.WriteLine("Started counting, please wait...");
    task.Wait(); // if you comment this line you will see that message "Finished counting" will be displayed before we actually finished counting.
    //Console.WriteLine("Finished counting to " + task.Result.ToString()); // using task.Result seems to also call task.Wait().
    Console.WriteLine("Finished counting.");
    Console.WriteLine("Press any key to exit program.");
    Console.ReadLine();
}

static async Task<long> asyncTest_count()
{
    long k = 0;
    Console.WriteLine("Started asyncTest_count()");
    await Task.Run(() =>
    {
        long countTo = 100000000;
        int prevPercentDone = -1;
        for (long i = 0; i <= countTo; i++)
        {
            int percentDone = (int)(100 * (i / (double)countTo));
            if (percentDone != prevPercentDone)
            {
                prevPercentDone = percentDone;
                Console.Write(percentDone.ToString() + "% ");
            }

            k = i;
        }
    });
    Console.WriteLine("");
    Console.WriteLine("Finished asyncTest_count()");
    return k;
}